Humoral immunity and cell-mediated immunity are the two major branches of the mammalian immune response. Humoral immunity involves the generation of antibodies to foreign antigens. Antibodies are produced by B-lymphocytes. Cell-mediated immunity involves the activation of T-lymphocytes which either act upon infected cells bearing foreign antigens or stimulate other cells to act upon infected cells. Both branches of the mammalian immune system are important in fighting disease. Humoral immunity is the major line of defense against bacterial pathogens. In the case of viral disease, the induction of cytotoxic T lymphocytes (CTLs) appears to be crucial for protective immunity. Thus, an effective vaccine preferably stimulates both branches of the immune system to protect against disease.
Vaccines present foreign antigens from disease causing agents to a host so that the host can mount a protective immune response. Often, vaccine antigens are killed or attenuated forms of the microbes which cause the disease. The presence of non-essential components and antigens in these killed or attenuated vaccines has encouraged considerable efforts to refine vaccine components including developing well-defined synthetic antigens using chemical and recombinant techniques. The refinement and simplification of microbial vaccines, however, has led to a concomitant loss in potency. Low-molecular weight synthetic antigens, though devoid of potentially harmful contaminants, are often not sufficiently immunogenic by themselves. These observations have led investigators to add immune system stimulators known as adjuvants to vaccine compositions to potentiate the activity of the vaccine components.
Immune adjuvants are compounds which, when administered to an individual or tested in vitro, increase the immune response to an antigen in a subject to which the antigen is administered, or enhance certain activities of cells from the immune system. A number of compounds exhibiting varying degrees of adjuvant activity have been prepared and tested (see, for example, Shimizu et al. 1985, Bulusu et al. 1992, Ikeda et al. 1993, Shimizu et al. 1994, Shimizu et al. 1995, Miyajima et al. 1996). However, these and other prior adjuvant systems often display toxic properties, are unstable and/or have unacceptably low immunostimulatory effects.
Presently, the only adjuvant licensed for human use in the United States is alum, a group of aluminum salts (e.g., aluminum hydroxide, aluminum phosphate) in which vaccine antigens are formulated. Particulate carriers like alum reportedly promote the uptake, processing and presentation of soluble antigens by macrophages. Alum, however, is not without side-effects and is unfortunately limited to humoral (antibody) immunity only.
The discovery and development of effective adjuvant systems is essential for improving the efficacy and safety of existing and future vaccines. Thus, there is a continual need for new and improved adjuvant systems, particularly those that drive both effector arms of the immune system, to better facilitate the development of a next generation of synthetic vaccines. The present invention fulfills these and other needs.